Switch to new regexp library

For now, this requires https://github.com/kemurphy/regexp and expects the generated libraries (regexp{,_macros}xxxxxx.{so,rlib}) in the same directory as the Makefile.  I have an open pull request at BurntSushi/regexp#2 for a small fix needed in the regexp! macro; hopefully that gets merged without issue and makes it into the rust repo when @alexcrichton merges it there.